The invention relates to a method of determining mechanical parameters of an object to be examined, which method includes the steps of:
a) generating mechanical oscillations in the object to be examined,
b) exciting the nuclear magnetization in conjunction with a magnetic gradient field which is synchronized with the mechanical oscillations, and receiving the MR signals arising in the object to be examined in order to produce an MR phase image,
c) changing the direction of the gradient of the gradient field and/or the phase difference between the mechanical oscillations and the gradient field,
d) repeating the steps a to c) a number of times,
e) determining, on the basis of the MR phase images, the deflection of the nuclear spins from their state of equilibrium which is caused by the mechanical oscillations, and calculating at least one mechanical parameter in dependence on the deflection.
Methods of this kind are known as MRE methods (MRE=Magnetic Resonance Elastography). Such a method utilizes the fact that the phase in an MR image of the object to be examined changes due to the mechanical oscillations active therein. The extent of such changes is dependent on the deflection (i.e. the shift out of the state of equilibrium) caused by the mechanical oscillation. Information concerning given mechanical parameters of the tissue, for example concerning the elasticity, can thus be derived from the MR phase images, i.e. images reproducing the phase of the nuclear magnetization.
EP-A 708 340 discloses an MR elastography method of this kind. Therein, first two MR phase images are formed of a slice of the object to be examined. The direction of the gradient of a magnetic gradient field which is synchronized with the mechanical oscillations is the same in both images, but the phases of this periodic gradient are 90xc2x0 offset in relation to the mechanical oscillation. Subsequently, further pairs of MR phase images are produced in which the periodic gradient extends perpendicularly to the gradient direction for the first pair. Subsequently, the direction of the mechanical oscillation in the object is changed and further sets of MR phase images are acquired.
The wavelength for the various pixels can be determined from each pair of MR phase images. The modulus of elasticity (Young""s modulus) can be calculated from the wavelength when the propagation speed of the wave in the object to be examined and the density thereof are known.
In another method which is known from Proceedings of ISM 1997, p. 1905, Vancouver, the phase of the deflection is determined from a series of MR phase images and therefrom the modulus of elasticity is calculated for each individual pixel.
It is a common aspect of the known methods that they yield satisfactory results only if no reflections occur in the object to be examined and if only transverse oscillations propagate in the object. However, in a real object to be examined, for example the body of a patient, reflections are inevitable and a purely transverse wave propagation cannot be achieved either. Moreover, it is known that longitudinal mechanical oscillations can penetrate deeper into a body, so that it would be desirable per se to achieve longitudinal propagation for an as large as possible part of the mechanical vibration energy.
It is an object of the present invention to conceive a method of the kind set forth in such a manner that the mechanical parameters of the object can be reliably determined also in the case of longitudinal wave propagation or reflections in the object to be examined.
This object is achieved according to the invention by determining the amount and the phase of the deflection in a three-dimensional zone for three mutually perpendicular directions and by calculating at least one mechanical parameter from these values of the deflection and from their spatial derivatives in at least a part of the three-dimensional zone.
The invention is based on the recognition of the fact that the propagation of mechanical waves in visco-elastic media can be described by a partial differential equation whose solution for each voxel is determined by the amount and phase of the deflection in three mutually perpendicular directions as well as by the spatial derivatives of the deflection. When these quantities have been determined for each voxel, the measured values can be inserted in the differential equation and at least one of the mechanical parameters contained in said equation can be calculated therefrom.
Thus, it does not suffice to determine the deflection for only a single direction in space. It is not sufficient either to determine the deflection in all three spatial directions for one slice only, not even if a mechanical parameter (for example, the modulus of elasticity) is to be determined for this slice only. This is because, as is known, a spatial derivative of the deflection in the direction perpendicular to the slice can be determined only if the deflection is determined also in zones outside the slice. The deflection, therefore, must be determined in a three-dimensional zone, i.e. the MR phase images should reproduce the spatial distribution of the phase of the nuclear magnetization in this three-dimensional zone.
When the mechanical oscillations act on the object to be examined in such a manner that essentially longitudinal oscillations occur in conformity with claim 2, a larger penetration depth is obtained for the oscillations so that the mechanical parameters, for example of the tissue in a human body, can be determined in a larger zone.
The version of the invention as disclosed in claim 3 ensures that during each repeated excitation of the nuclear magnetization the mechanical oscillations are associated exactly in time with the gradient fields produced in conjunction with the excitation of the nuclear magnetization, notably with the gradient field synchronized with the mechanical oscillations.
The invention is based on the assumption that a series of MR phase images is formed of a three-dimensional zone so that comparatively long measuring times occur. In order to ensure that these measuring times are not prolonged further by waiting for the decaying of the nuclear magnetization in the excited zone after an excitation, the slices constituting the three-dimensional zone to be excited are excited by means of a multi-slice method as disclosed in claim 4.
A preferred version of the invention is disclosed in claim 5. Even though other mechanical parameters can also be calculated, for example, the density of the tissue, the Poisson number or the attenuation of the wave by the tissue, the modulus of elasticity is the most relevant parameter for the diagnosis. The elasticity is the mechanical parameter to be determined by a physician during the palpation of the tissue.
Particularly advantageous is the determination of the modulus of elasticity during examination of the mamma in conformity with claim 6.
Claim 7 describes a device for carrying out the method according to the invention.
In accordance with claim 8 the wave and the magnetic gradient field synchronized therewith vary sinusoidally in time. Even though a different periodic variation is also feasible, for example a sawtooth, delta or squarewave variation, the sinusoidal variation offers advantages. As many as 6 different mechanical parameters can be calculated on the basis of the measurement results.
The further embodiment in conformity with claim 9 enables calculation of more than 6 different mechanical parameters. In accordance with claim 10 the modulus of elasticity and one further mechanical parameter, for example, the attenuation coefficient, can then be calculated, even when one of these two parameters (or both parameters) is (are) not an isotropic quantity.
Claim 11 describes a computer program which is suitable for the method according to the invention.